Conventionally, numerous wavelength conversion laser light sources for obtaining a visible laser beam of green light or the like by performing wavelength conversion to light that is emitted from a Nd:YAG laser or Nd:YVO4 laser by using the nonlinear optical effect, or obtaining an ultraviolet laser beam by further converting the green light have been developed and put into practical application. The foregoing converted light is used for laser processing, laser display and other purposes.
FIG. 1 shows a standard configuration example of a conventional wavelength conversion laser light source using the nonlinear optical effect. In order to obtain the nonlinear optical effect, it is necessary to use a nonlinear optical crystal with birefringence. Specifically, as nonlinear optical crystals with birefringence, LiB3O5 (lithium triborate: LBO), KTiOPO4 (potassium titanyl phosphate: KTP), CsLiB6O10 (cesium lithium borate: CLBO), LiNbO3 (lithium niobate: PPLN) formed with a periodically poled structure, LiTaO3 (lithium tantalate: PPLT), and the like have been used.
As shown in FIG. 1, a wavelength conversion laser light source 100 includes a fundamental light source 101, a condensing lens 108, a wavelength conversion element (nonlinear optical crystal) 109, a re-collimating lens 111, a wavelength separating mirror 113, a temperature holding device 116 such as a heater for maintaining the temperature of the wavelength conversion element 109 constant, a control device 115 for controlling the laser output, and a temperature controller 122 for controlling the temperature of the nonlinear optical crystal disposed in the control device 115. As the fundamental light source 101, Nd:YAG laser or Nd:YVO4 laser with a wavelength of 1.06 μm, or a fiber laser using Yb-doped fiber is often used.
Here, the actual operation is explained upon taking the second harmonic generation for generating a laser beam with a half wavelength of 0.532 μm from the laser beam with a wavelength of 1.06 μm as an example.
The laser beam with a wavelength of 1.06 μm emitted from the fundamental light source 101 is condensed on the nonlinear optical crystal 109 by the condensing lens 108. Here, the refractive index of the nonlinear optical crystal 109 relative to the wavelength of 1.06 μm needs to match the refractive index relative to the light with a wavelength of 0.532 μm to be generated. This is referred to as phase matching. Generally speaking, since the refractive index of a crystal changes with the crystal's temperature conditions, it is necessary to keep the crystal temperature constant. Thus, the nonlinear optical crystal itself is disposed in the temperature holding device 116 and the temperature is held according to the type of crystal.
For example, if the phase matching method referred to as type-1 noncritical phase matching is adopted using an LBO crystal, the crystal needs to be held at a temperature of 148° C. to 150° C.
Moreover, when using the LiNbO3 crystal with a periodically poled structure, the temperature and wavelength to be subject to phase matching can be arbitrarily decided by designing the period of the periodically poled structure. However, in order to continue maintaining the phase matching conditions, the element temperature and the fundamental wavelength must be kept constant (refer to Patent Document 1 and Patent Document 2).
FIG. 2 schematically shows the control loop of monitoring the green light as the wavelength converted light, and controlling the output to be constant.
The control loop 250 shown in FIG. 2 controls the fundamental light output 260 from the fundamental light source 101 by controlling the inrush current 240 to the fundamental light source 101. The fundamental light output 260 enters the wavelength conversion element 109 configured from a nonlinear optical crystal with a constant temperature based on the control of the element temperature control unit 280. After being subject to wavelength conversion in the wavelength conversion element 109, green light 270 is output from the wavelength conversion element 109. In order to make the output of the green light 270 constant, the method of controlling the current 240 to be incident into the fundamental light source 101 based on the control loop 250 in accordance with the light intensity of the green light 270 has been used as APC (Auto Power Control).
Meanwhile, when performing wavelength conversion using the nonlinear optical effect, it is necessary to satisfy the phase matching conditions. Thus, the polarization direction of the fundamental light that is emitted from the fundamental light source 101 and the wavelength of the fundamental light that is emitted from the fundamental light source 101 are also important factors of wavelength conversion.
Patent Document 3 shows a method of reducing noise of the output upon performing relaxation oscillation by monitoring the respective outputs of the fundamental light and the second harmonic light.
Meanwhile, Patent Document 4 proposes a method of acquiring the respective polarization elements of the fundamental light and feeding back the same to the driving of the phase difference adjustment means provided in the resonator in a wavelength conversion laser light source using a wavelength conversion element which performs type-II phase matching.
Nevertheless, with the method of Patent Document 4, it is difficult to stabilize the output since the phase matching state of the wavelength conversion element will jointly fluctuate.
With a conventional control loop, in addition to the problem that it cannot be used in the state of relaxation oscillation as indicated from the past, there is also a problem in that it is unable to deal with changes in polarization or changes in wavelength even in a steady state.
As described above, it has been discovered that stable and efficient wavelength conversion cannot be achieved due to changes in the phase matching state of the wavelength conversion element caused by the state of the fundamental light.